Known plasma or reactive ion etching (RIE) processes for selectively etching silicon dioxide (or PSG) layers on semiconductor chips require heretofor inadequate trade-offs between chamfering of the photoresist, etch rate selectivity between the SiO.sub.2 layer and an etch stop, and "RIE lag" (the phenomenon where large area via holes are etched more rapidly than small area via holes on the same chip). Such RIE processes are typically optimized for a particular use by varying the chemistry, pressure, flow rate, and power parameters of the environment.
High pressure (i.e., pressures greater than 1 Torr) fluorocarbon RIE processes are desirable in that they provide good selectivity of silicon dioxide. Such processes include, for example, CHF.sub.3 +CF.sub.4 +Ar, and CHF.sub.3 +He chemistries. However, these high pressure processes have been found to provide particularly poor RIE lag characteristics.
Low pressure fluorocarbon processes are desirable for selective etching of silicon dioxide, in that they provide less RIE lag than the high pressure processes described above. Such processes include, for example CHF.sub.3 +He chemistry at 0.5-100 mTorr pressure. In fact, such processes function adequately in low density, low power, batch reactors. Such processes, however, provide unacceptable chamfering of photoresist mask layers when used in higher power, single wafer reactors. This chamfering results in loss of critical dimensions, requiring the use of undesirably large ground rules.
Low pressure, high density processes, which utilize plasma generation mechanisms such as magnetrons, inductive coupling, or electron cyclotron resonance, reduce the photoresist mask chamfering typically found with the lower pressure processes. However, this class of RIE processes provides poor selectivity to the silicon dioxide, and may result in polymer-induced RIE lag within small via openings.
Anisotropic silicon etching (whereby near-vertical etched silicon profiles are achieved) using NF.sub.3 or SF.sub.6 as a primary etchant, and CHF.sub.3 and N.sub.2 as additives, are known in the art. See U.S. Pat. No. 4,741,799, issued on May 3, 1988, to Lee Chen et al. and assigned to the present assignee. This particular etching system is useful for selectively etching silicon through a silicon dioxide mask.
In U.S. Pat. No. 4,767,724, issued on Aug. 30, 1988, to Manjin J. Kim et al., a mixture of CHF.sub.3 and argon is mentioned as "not the preferred etching gas" for etching vias in silicon dioxide down to an aluminum oxide etch-stop layer. For high selectivity between the etching rates of SiO.sub.2 and Al.sub.2 O.sub.3, Kim et al. recommends a gas mixture of NF.sub.3 and argon.
Nitrogen trifluoride with argon, but without a carbon-containing gas such as CHF.sub.3, is shown in U.S. Pat. No. 4,904,341, issued on Feb. 27, 1990, to Richard D. Blangher et al., for etching SiO.sub.2 while avoiding unwanted polymer by-products which carboncontaining etch gases are prone to leave on the circuit workpiece and on the walls of the reactor.
Etching of oxide selectively to tantalum with mixtures of fluorine containing and nitrogen containing molecules is taught in Japanese Kokai No. 1-33323.
While various high and low pressure, fluorocarbon-based, RIE processes are known in the art, all of these processes require an unacceptable trade-off between selectivity, mask chamfering, and RIE lag. No process is known for selectively etching SiO.sub.2 (or PSG) which simultaneously yields acceptable photoresist chamfering, high selectivity, and good (i.e. low) "RIE lag" characteristics.